Those concerned with the development of radar systems have engaged in a continuing search for systems with both long detection range and fine range resolution. Ideally, both goals may be accomplished by the transmission of extremely narrow pulses of exceptionally high peak power. However, there are practical limits on the level of peak power which can be generated, especially in a portable radar set-up. To obtain long detection ranges at pulse repetition frequencies (PRF) low enough for pulse delay ranging, fairly wide pulses must be transmitted.
So-called pulse compression techniques present one solution to the problem. Internally modulated pulses of sufficient width to provide the necessary average power at a reasonable level of peak power are transmitted. When the echoes are received they are "compressed" by decoding or stripping their modulation.
A variety of pulse compression techniques is described in: Skolnik, "Introduction to Radar Systems" McGraw Hill, 1980, pp. 420-434. The two most common methods of coding are linear frequency modulation (chirping) and binary phase or polyphase modulation. In chirp modulation, the radio frequency of each transmitted pulse is increased at a constant rate throughout its length. Every echo, consequently, has the same linear increase in frequency. In phase modulation, each transmitted pulse is, effectivley, marked off into narrow segments of equal length. The phase of certain segments is shifted by 180.degree. according to a predetermined binary code. In polyphase coding, a variety of different harmonically related phases, e.g. 0.degree., 180.degree., 270.degree., etc., is used.
A variety of methods have been developed for decoding the received echo and assigning the target to a specific range bin. One device used for decoding purposes is the time-integrating correlator. Operation of time integrating correlators is discussed in: B. J. Darby, et al. "The Tapped Delay Line Active Correlator: A Neglected SAW Device," 1975, IEEE Ultrasonics Symposium, IEEE Cat. No. 75CHO994-4SU, pp. 193.varies.196, September 1975.
Briefly, a SAW time-integrating correlator is composed of a multi-tapped SAW delay line, a balanced mixer array, and an integrator array. The outgoing signal carrier is modulated, as mentioned before, by the desired code at the time of transmission. A reference carrier (which is inputted to the mixer array) is modulated by the same (but delayed) code when the echo signal has returned. The echo signal is then interrogated after passage through the SAW delay line by the mixer array. The reference code modulation is applied at the time of range interrogation .tau..sub.R. The reference carrier is not initialized at the time of signal transmission, .tau..sub.S, but at the delay time of interrogation of a particular signal return, .tau..sub.R. Consequently, the reference signal and the transmitted signal carriers are not phase-coherent for arbitrary delays.
The lack of coherence between the reference signal and the received echo results in an arbitrary phase factor of 2.pi.f(.tau..sub.R -.tau..sub.S) where f is the carrier frequency. This results in a sinusoidal amplitude variation in the response of the time integrating correlator of the form cos (2.pi.(.tau..sub.R -.tau..sub.S) ) which produces false negatives, i.e. a target will not be detected except when the sinusoidal response is at its maximum. Clearly, this amplitude variation must be eliminated in order to detect returns at arbitrary differential time delays, .tau..sub.R -.tau..sub.S.
Those concerned with the development of radars using time integrating correlators are concerned with the problems of false negative responses and have continually sought for methods in apparatus which will improve the probability of detection of real targets through elimination of the problem of non-coherence between reference and transmitted signals.